This invention relates to single-crystal articles made of nickel-base superalloys, and, more particularly, to such articles whose compositions are modified with additions of hafnium and/or zirconium to achieve improved properties.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys and can operate at temperatures of up to 1900-2100xc2x0 F.
Many approaches have been used to increase the operating temperature limits and operating lives of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. The articles may be prepared as oriented single crystals to take advantage of superior properties observed in certain crystallographic directions. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation. Protective coatings may be applied to the surfaces of the turbine blades and vanes.
Specific alloys have been developed for use in single-crystal turbine blades and vanes. Examples include nickel-base superalloys known as Renexe2x80x2 N5, Renexe2x80x2 N6. CMSX-4. CMSX-10, PWA 1480, PWA 1484, and MX-4.
These superalloys exhibit improved properties as compared with conventional alloys, but there is always a need for further improvements to the strengths, elevated temperature capabilities, operating lifetimes, and stabilities of the alloys used in single-crystal article applications. The present invention fulfills this need, and further provides related advantages.
The present invention provides nickel-base superalloy single crystal articles having compositions that exhibit improved mechanical properties for high-temperature applications. The invention is therefore applied most beneficially to articles used in high-temperature applications, such as aircraft gas turbine blades and vanes. The alloy modifications to the nickel-base superalloys are selected so that other properties of the alloys, such as castability and heat treatability, are not adversely affected. The alloy of the invention is also compatible with the use of both diffusion and overlay protective coatings and thermal barrier coatings.
An article comprises substantially a single crystal. The article has a composition, in weight percent, consisting essentially of (a) a modifying element selected from the group consisting of from about 0.2 to about 2.0 percent by weight hafnium, and from about 0.1 to about 0.5 percent by weight zirconium, and combinations thereof, and (b) a base alloy composition of from about 4 to about 20 percent cobalt, from about 1 to about 10 percent chromium, from about 5 to about 7 percent aluminum, from 0 to about 2 percent molybdenum, from about 3 to about 8 percent tungsten, from about 4 to about 12 percent tantalum, from 0 to about 2 percent titanium, from 0 to about 8 percent rhenium, from 0 to about 6 percent ruthenium, from 0 to about 1 percent niobium, from 0 to about 0.1 percent carbon, from 0 to about 0.01 percent boron, from 0 to about 0.1 percent yttrium, balance nickel and incidental impurities.
The hafnium and/or zirconium modifying elements are added to the base alloy composition in a specific narrow range such that the benefits of their increased levels on the mechanical properties of the article are not overshadowed by any adverse effects on other properties such as castability, stability, and/or heat treatability. The modifying element is present in the superalloy composition in an amount of from about 0.2 to about 2.0 percent by weights preferably about 1.0 percent by weight, for the case of hafnium; and/or in an amount of from about 0.1 to about 0.5 percent by weight, preferably about 0.25 percent by weight, for the case of zirconium. Combinations of hafnium and zirconium within these ranges are operable.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.